1. Field of the Invention
This invention relates to a toner image fixing device.
2. Description of the Prior Art
In a field so as electrophotography which deals with toner images, a device designed such that a toner image supporting member is conveyed while being held between a pair of rollers at least one of which is heated from the inside or the outside thereof whereby toner is heated and melted and fixed on the supporting member has often been used as the device for fixing a toner image on the supporting member and in that case, in order to prevent offset of the toner and twisting of the supporting member about the roller, it is well-known to apply offset preventing liquid (usually, silicone oil) to the fixing roller against which the toner image bearing surface of the supporting member is urged.
The device heretofore practically used to apply the offset preventing liquid to the fixing roller is a device in which felt having absorbed and retained the silicone oil by capillary phenomenon is brought into direct contact with the fixing roller or brought into contact with an applicator roller rotating while being in contact with the fixing roller or one roller of an applicator roller train comprising a plurality of rollers rotating while being in contact with each other, whereby the silicone oil is supplied and applied to the fixing roller. In such device, however, too much oil tends to be applied and therefore, the toner image supporting member is stained with the oil and later, when ink is applied to the supporting member, the ink does not spread well and in addition, slippage occurs between the fixing roller and the pressing roller to disturb the toner image. Also, a greater amount of oil than necessary is uneconomically consumed and this has led to a high frequency with which the oil is supplied or the oil supply cartridge is replaced by a new one, which in turn has meant cumbersomeness of maintenance. U.S. Pat. No. 3,718,116, U.S. Pat. No. 3,745,972 and Japanese Utility Model Publication No. 51168/1978 disclose devices which use two-layer felt comprising felt of thin fiber density and felt of thick fiber density and in which the felt of thick fiber density is brought into contact with the roller to reduce the amount of oil applied. In these devices, however, the amount of oil applied is still great and a greater amount of oil than necessary is applied to the roller to prevent the offset and the twisting of the supporting member about the roller. Moreover, in the above-described devices, streak-like application irregularities tend to occur and it is very difficult to uniformly apply a small amount of oil.
Another serious disadvantage of the above-described devices in which felt is brought into contact with the roller to apply the offset preventing liquid to the roller is that toner or paper powder or carrier particles contained in the latent image developer forming the toner image clog the felt, whereby application irregularities are often caused. If such application irregularities occur, the offset of the toner partly occurs and the image supporting member twists about the roller and in addition, where the fixing roller and the pressing roller are covered with a parting material such as silicone rubber having an offset preventing liquid absorbing characteristic, there is an inconvenience that said application irregularities result from a difference in the amount of swell between the rollers and wrinkles tend to occur to the toner supporting member. Further, when a great amount of toner adheres to said felt, this will damage the roller and further expedite the aforementioned application irregularities, thus causing various unsatisfactory fixation effects.
On the other hand, a method is conceivable in which a silicone rubber roller having its rubber structure impregnated with silicone oil or a roller comprising an apertured pipe containing therein silicone oil and covered with silicone rubber is brought into contact with the fixing roller to apply offset preventing liquid to the fixing roller through the rubber structure. According to such a method, uniform application of a slight amount of oil would be possible, but the amount of oil applied would be too slight and this would be entirely unsuitable for a device in which seven or eight or more sheets of A4 size paper must be processed for fixation per minute, such as the fixing devices used in the present-day copying apparatus. Further, in this method, as the viscosity of the liquid is higher, the amount of liquid applied would be further reduced.
As a device which provides a means to overcome the above-noted various inconveniences, there are the fixing devices disclosed in Japanese Laid-open Patent Application Nos. 110049/1977 and 37752/1979. The fixing devices described in these two laid-open patent applications are designed such that the amount of silicone oil applied to the fixing roller is controlled by non-fibrous high-molecular synthetic resin having fine continuous pores, namely, not a material such as felt comprising entangled fibers or a sponge having a great quantity of air bubbles dispersed in synthetic resin material, but a high-molecular synthetic resin material having a great number of fine pores extending from the front surface of the synthetic resin material to the back surface thereof. For such material, tetrafluoroethylene having continuous pores such as GORE-TEX (produced by W. L. GORE & ASSOCIATES, INC., tradename) or FLUOROPORE (produced by Sumitomo Denki-Kogyo K. K., tradename) is preferable. In the devices of the aforementioned two laid-open patent applications using such synthetic resin material, the fine continuous pores are not clogged with toner or paper powder and accordingly, there occurs no inconvenience that the amount of oil applied is decreased with time and application irregularities begin to occur. It has been confirmed by the inventor that, in the case of the devices described in said two laid-open patent applications, both high-viscosity liquid and low-viscosity liquid are controlled to an appropriate amount and uniformly applied in the central area of said synthetic resin material with respect to the lengthwise direction of the roller while, in the opposite end portions thereof, the amount of liquid applied tends to be great. This is considered to be attributable to the fact that when the applicator is urged against the roller, the offset preventing liquid contained in the applicator tends to move outwardly in the lengthwise direction of the roller at the end portions of the synthetic resin material with respect to the lengthwise direction of the roller. In any case, if the amount of liquid applied to the roller is great at the end portions of the synthetic resin material with respect to the lengthwise direction of the roller, this may result in slippage between the fixing roller and the pressing roller, swell irregularity of the roller covered with silicone rubber, liquid-contamination of the toner image supporting member, etc. and in addition, a greater amount of liquid than necessary may be uneconomically consumed.